There is about 17 months between my two. We had the new baby in a bassinette for 3 months in our room
and found Dd1 was ready for a bed by the time dd2 needed the cot. I wasn't sure either but playing it by ear worked well for us and I didn't end up buying a second cot.

Dd2 however is only just ready now for a bed at almost 2, so I think it really depends on the child.